Until recent years the mapping of the shape of the visual response field of an individual involved crude techniques that rendered the results fraught with imprecision and inaccuracies. This visual field mapping was of limited utility in medical diagnosis, especially regarding diseases of the eye and brain.
However, with the advent of modern visual field mapping devices, the visual field can be mapped accurately and precisely. As a result this technique has become an important diagnostic tool. These devices provide a concave screen on which a large plurality of test points are disposed. The test points may comprise individual LED's, or may be joined to the end of an individually illuminated light conducting fiber. The test points are randomly and sequentially illuminated, and the subject's response is recorded automatically on a chart or other permanent record. The luminance of the screen and of the illuminated test points are controlled at all times so that a mapping procedure may be repeated with a great deal of accuracy. Thus small changes in the subject's visual field may be detected, and related diseases may be diagnosed earlier. One exemplary device is described in U.S. Pat. No. 4,146,311, issued to William C. Murr on Mar. 27, 1979.
One minor problem in visual field testing is that the test points disposed about the screen may not have the same luminance as the screen itself. This situation may arise because the LED's or optical fiber ends may reflect less of the light illuminating the screen (when they are not actuated). As a result the subject actually has an effective stimulus which is greater than would be experienced if these areas were of uniform luminance with the surrounding screen. That is, the subject sees the test points as dark spots which become lighted when actuated, rather than seeing a completely neutral field in which points of light appear randomly. This effect is most noticeable at low stimulus values.